This project aims to characterize the unfolding of individual differences in emotional reactivity during the course of development in the rat. It will focus both on the emergence of these differences in emotionality at the behavioral level, and on the development of neuronal differences in gene expression that accompany the distinct patterns of behavior. We propose to study how stress-related and monoaminergic (serotonergic and noradrenergic) genes are differentially expressed across the developmental trajectory of two types of animals that show spontaneous differences in emotional reactivity: High Responders (HR) that exhibit a higher rate of exploration of a novel environment and lower levels of anxiety-like behavior, and Low Responders (LR) that exhibit the opposite behavioral phenotype. We will ask how these behavioral and neuronal differences are modulated by maternal behavior and by the family background of the animals (i.e. whether their parents or themselves HR, LR or a combination). We have already shown that the mothers behave differently towards their offspring depending on whether they themselves are HR or LR. We have also shown that offspring are different both as a function of the classification of their fathers and their mothers. We will use cross-fostering to separate the biological and behavioral contributions of the mother to the offspring's emotional reactivity. The interplay between maternal behavior and family background in determining the phenotype of the offspring will e a major focus. Finally, towards the end of this project, we will study the expression of selected "novel candidate genes" discovered under Project 1 (Subproject 0009) and related their developmental pattern to the unfolding of differences in emotional responsiveness. We hope that these developmental studies will add to our understanding of individual differences in emotionality by revealing the sequence of neurobiological events and the mat3ernal influences that lead to the distinctive behaviors.